Bake sound to F Curve giving square curves?
The issue was that in setting up the custom property 'Sound'. In the video, the tutor enters '0.0' into the Property Value. I didn't notice the decimal and entered '0', thus switching the property to 'integer' instead of the desired 'float'.

I removed the keyframes from the Sound property, edited the Property Value to '0.0', added a keyframe and reimported the sound (bake sound to fcurve).